The invention relates generally to energy recovery systems and more particularly to an air heating system and method of operating same which recover energy from flue gases and the like and provide such heated air at a substantially constant temperature.
Dwindling reserves and increasing expense of fossil fuels have prompted a reexamination of nearly every aspect of mechanical and heat energy generation, utilization and ultimate disposition. Whereas the aspects of energy conversion such as fossil fuel into heat and the utilization of such energy have come under close scrutiny in recent years, a similar investigation with regard to the ultimate disposition of waste energy, i.e., energy not utilized in a given process or for an intended purpose, has occurred only to a significantly lesser extent.
Apparatus is known for the recovery of heat from exhaust flues and the like. For example, U.S. Pat. No. 2,895,719 discloses a method and apparatus for recovering heat from a fluid medium such as a stream of hot exhaust gas. In the device disclosed, high velocity gases create a condition which improves heat transfer to the flow of heated fluid. This device, however, is without apparatus for maintaining a constant output temperature and therefore the output temperature varies as a function of the volumes of heating and heated fluid and the inlet temperatures of the heated and heating fluid.
U.S. Pat. No. 4,050,627 discloses another heat recovery system for flues having a pair of independent passages in thermal communication. An adjustable baffle plate determines the proportion of the heat exchanger exposed to the flue gases and thereby adjusts the maximum energy transfer capability of the system. A thermostatically operated fan is also disclosed to provide forced air circulation through the system.
One method of temperature control known in the prior art is the utilization of a pair of adjustable dampers disposed in parallel in a flow path and associated respectively with a heating and cooling means or heating and non-heating means. The dampers are mechanically connected such that as one damper opens and permits flow therethrough the other damper closes. Temperature control schemes incorporating this approach are disclosed in U.S. Pat. Nos. 3,122,202, 3,139,020 and 3,650,318. From an entropy standpoint, such mixing systems are now appreciated to be inefficient and are therefore often considered to be undesirable.
An energy recovery system incorporating a bypass flow control is disclosed in U.S. Pat. No. 2,570,859. Here, exhaust gases are utilized to preheat outside air utilized for combustion. A control system monitors the temperature of the exhaust air exiting the heat exchanger in order to maintain it above a certain minimum temperature to inhibit condensation in the exhaust stack. The temperature controller is an averaging or totalizing type which also monitors the temperature of the air supplied to the inlet side of the heat exchanger and provides a controlling signal based upon the average temperature of the air to be heated prior to its passage through the heat exchanger and the temperature of the exhaust gas subsequent to its exit from the heat exchanger. This temperature signal is utilized to control not only bypass of inlet air around the heat exchanger but also recirculation of air through the heat exchanger. Finally, a flow meter is also utilized which adjusts the amount of recirculated air.
It is apparent from the foregoing that although apparatus has been heretofore produced for assisting the recovery of heat from exhaust gases and similar flows of heated air which would otherwise be wasted, such structures do not include control systems for accurately maintaining the temperature of the output stream of heated air. This can be a significant problem in installations utilizing the recovered heat for processes which require constant temperatures since the supply of waste heat may fluctuate greatly as process steps such as increased engine speed, batch heating or cooling, or other machine cycling occurs. Such prior art also does not disclose systems wherein the volume of air to be heated by the heat exchanger is reduced in order to maintain a constant output temperature, for example, when heat input to the heat exchanger is reduced.